Dry adhesives and methods for making dry adhesives

ABSTRACT

Dry adhesives and methods for forming dry adhesives. A method of forming a dry adhesive structure on a backing material, comprises: forming a template backing layer of energy sensitive material on the backing material; forming a template layer of energy sensitive material on the template backing layer; exposing the template layer to a predetermined pattern of energy; removing a portion of the template layer related to the predetermined pattern of energy, and leaving a template structure formed from energy sensitive material and connected to the substrate via the template backing layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a Continuation In Part application of U.S.Non-provisional application Ser. No. 12/448,242 entitled “DRY ADHESIVESAND METHODS FOR MAKING DRY ADHESIVES” filed on Aug. 27, 2009, which is anational stage entry of Patent Cooperation Treaty internationalapplication serial number PCT/US2007/025683, entitled “DRY ADHESIVES ANDMETHODS FOR MAKING DRY ADHESIVES” filed on Dec. 14, 2007, which claimsthe benefit of U.S. Provisional Application Ser. No. 60/874,850, filedon Dec. 14, 2006, all are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

This invention was made, in part, with government support under GrantNumber CNS-0428738 awarded by the National Science Foundation. TheUnited States government may have certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to dry adhesives, and methods for makingdry adhesives including, for example, fibrillar microfibers andnanofibers.

BACKGROUND OF THE INVENTION

Fibrillar adhesives on the feet of geckos and spiders and other animalshave been of great interest because they can repeatedly attach to widerange of surfaces with a controllable adhesion strength in variousenvironments including vacuum, and leave no residue. Furthermore,fibrillar adhesives are self-cleaning which allows for long lifetime andrepeated use without significant performance degradation [W. Hansen andK. Autumn. Evidence for self-cleaning in gecko setae. Proceedings of theNational Academy of Sciences, 102:385-389, 2005.]. These foot-hairsconform to the surface roughness to increase the real contact area,resulting in high adhesion by surface forces [K. Autumn, Y. A. Liang, S.T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full.Adhesive force of a single gecko foot-hair. Nature, 405:681-685, 2000.].This adhesion, called dry adhesion, is argued to arise from molecularsurface forces such as van der Waals forces [K. Autumn, M. Sitti, Y. A.Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R.Fearing, J. N. Israelachvili, and R. J. Full. Evidence for van der waalsadhesion in gecko setae. Proceedings of the National Academy ofSciences, 99:12252-56, September 2002.], [K. Autumn, Y. A. Liang, S. T.Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full.Adhesive force of a single gecko foot-hair. Nature, 405:681-685, 2000.],possibly in combination with capillary forces [G. Huber, H. Mantz, R.Spolenak, K. Mecke, K. Jacobs, S. N. Gorb, and E. Arzt. Evidence forcapillarity contributions to gecko adhesion from single spatulananomechanical measurements. Proceedings of the National Academy ofSciences, 102: 16293-16296, 2005.], [V. Sun, P. Neuzil, T. Kustandi, S.Oh, and V. D. Samper. The nature of the gecko lizard adhesive force.Biophysical Journal: Biophysical Letters, L14-L17, 2005]. The complexityof the structure of these fibers differs among the species of animal.For large lizards such as the Tokay gecko the fibers take on acomplicated branched structure, whereas for lighter animals such asspiders and anoles the structure is a simple array of angled high aspectratio microfibers [E. Arzt, S. Gorb, and R. Spolenak. From micro to nanocontacts in biological attachment devices. Proceedings of the NationalAcademy of Sciences, 100(19):10603-06, September 2003.]. Some Geckospecies have adhesion strength capabilities as high as 100 kPa [K.Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R.Fearing, and R. J. Full. Adhesive force of a single gecko foot-hair.Nature, 405:681-685, 2000.]. In Geckos, the oriented fibers are made ofa stiff biomaterial (beta-keratin) with a Young's modulus ofapproximately 4 GPa [B. N. J. Persson. On the mechanism of adhesion inbiological systems. Journal of Chemical Physics, 118:7614-7621, April2003.] and have diameters from 0.2 to 5 μm [E. Arzt, S. Gorb, and R.Spolenak. From micro to nano contacts in biological attachment devices.Proceedings of the National Academy of Sciences, 100(19):10603-06,September 2003.]. The structure and material properties such as Young'smodulus allow the fibers to individually bend and adapt to a widevariety of surface roughnesses and also to return to their originalshape after release from the surface. Fabrication of similar syntheticstructures would enable the production of long-lifetime reusablefibrillar adhesives with broad applications.

The enhanced adhesion from fibrillar surfaces has been studied anddescribed in terms of fracture mechanics, elastic beam theory, andsurface interaction forces [A. Crosby, M. Hageman, and A. Duncan.Controlling polymer adhesion with “pancakes”. Langmuir, 21:11738-11743,2005.], [T. Tang, C.-Y. Hui, and N. J. Glassmaker. Can a fibrillarinterface be stronger and tougher than a non-fibrillar one? Journal ofThe Royal Society, Interface, 2(5):505-516, 2005.], [C. Hui, N. J.Glassmaker, T. Tang, and A. Jagota. Design of biomimetic fibrillarinterfaces: 2. mechanics of enhanced adhesion. Journal of The RoyalSociety, Interface, 1:35-48, 2004.], [B. N. J. Perrson and S. Gorb. Theeffect of surface roughness on the adhesion of elastic plates withapplication to biological systems. Journal of Chemical Physics,119(21):11437-11444, 2003.], [B. N. J. Persson. On the mechanism ofadhesion in biological systems. Journal of Chemical Physics,118:7614-7621, April 2003.], [N. J. Glassmaker, A. Jagota, C.-Y. Hui,and J. Kim. Design of biomimetic fibrillar interfaces: 1. makingcontact. Journal of The Royal Society, Interface, 1(1):23-33, November2004.], [J. Y. Chung and M. K. Chaudhury. Roles of discontinuities inbio-inspired adhesive pads. Journal of The Royal Society Interface,2:55-61, 2005.], including analysis of the effects of tip shape andfiber size [R. Spolenak, S. Gorb, H. Gao, and E. Arzt. Effects ofcontact shape on the scaling of biological attachments. Proceedings ofthe Royal Society A, 461:305-319, 2005.], [H. Gao and H. Yao. Shapeinsensitive optimal adhesion of nanoscale fibrillar structures.Proceedings of the National Academy of Sciences, 101(21):7851-7856, May2004.]. Work has also been conducted to create synthetic fiber adhesivesvia various fabrication techniques. Since van der Waal's forces areuniversal, a wide variety of materials and techniques may be used toconstruct the fibers. Methods such as electron-beam lithography [A. K.Geim, S. V. Dubonos, I. V. Grigorieva, K. S. Novoselov, A. A. Zhukov,and S. Y. Shapoval. Microfabricated adhesive mimicking gecko foot-hair.Nature Materials, 2:461-463, 1 Jun. 2003.], micro/nanomolding [N. J.Glassmaker, A. Jagota, C.-Y. Hui, and J. Kim. Design of biomimeticfibrillar interfaces: 1. making contact. Journal of The Royal Society,Interface, 1(1):23-33, November 2004.], [M. Sitti and R. Fearing.Synthetic gecko foot-hair micro/nanostructures as dry adhesives. Journalof Adhesion Science and Technology, 17(5):1055-74, May 2003.], [C.Majidi, R. Groff, and R. Fearing. Clumping and packing of hair arraysmanufactured by nanocasting. Proc. of the ASME International MechanicalEngineering Congress and Exposition, 579-584, 2004.], [C. Menon, M.Murphy, and M. Sitti. Gecko inspired surface climbing robots. Proc. ofthe IEEE Int. Conf. on Robotics and Biomimetics, pages 431-436, August2004.], and self-assembly are employed to fabricate fibers from polymers[K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S.Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full.Evidence for van der waals adhesion in gecko setae. Proceedings of theNational Academy of Sciences, 99:12252-56, September 2002.], [M. Sittiand R. Fearing. Synthetic gecko foot-hair micro/nanostructures as dryadhesives. Journal of Adhesion Science and Technology, 17(5):1055-74,May 2003.], polymer organorods [M. T. Northen and K. L. Turner. A batchfabricated biomimetic dry adhesive. Nanotechnology, 16:1159-1166,2005.], and multi-walled carbon nanotubes [Y. Zhao, T. Tong, L. Delzeit,A. Kashani, M. Meyyappan, and A. Majumdar. Interfacial energy andstrength of multiwalled-carbon-nanotube-based dry adhesive. Journal ofVacuum Science 4 Technology B: Microelectronics and NanometerStructures, 24:331-335, 2006.], [B. Yurdumakan, N. R. Raravikar, P. M.Ajayanb, and A. Dhinojwala. Synthetic gecko foot-hairs from multiwalledcarbon nanotubes. Chemical Communications, page 3799-3801, 2005.].

U.S. Pat. No. 6,872,439 describes a variety of methods for thefabrication of microfibers, including the fabrication of angledmicrofibers. The methods include the fabrication of negative templatesby using substrates with fabricated or self-organized high aspect ratioholes. These holes can be made by imprinting the desired shape usingsingle sharp probes, or by using optical lithography, deep reactive ionetching (DRIE) with thermal oxidization processing, black siliconetching, laser micro/nanomachining, electron-beam lithography,nano-imprinting, or soft-lithography. A second approach is through theuse of a positive template that is fabricated by molding alreadyexisting or fabricated high aspect ratio stiff micro/nano-structuresthat are not appropriate to use directly as synthetic hair. Thesemicro/nano structures, for instance, could be carbon nanotubes,nanowires, or nanorods.

Several methods are disclosed to achieve oriented fibers. First, a softsurface, such as wax, may be indented by a sharp probe at an angle.Another method is to shear a molded template under stress and at aspecific temperature to plastically deform it to a desired angle .theta.A third method for orienting the fibers is the dry etching (e.g. DRIE)of an inclined silicon wafer. But, DRIE can only etch vertical wallseven the wafer is tilted.

U.S. patent application Ser. No. 10/863,129 (published as US2005-0271869 A1) and Ser. No. 10/982,324 (published as US 2005-0271870A1) disclose a method for forming hierarchical structures of microfiberswith smaller microfibrils attached to the end. In one embodiment, theseapplications describe a method to fabricate nanostructures that areangled. This method relies on the insertion of oriented fibers into aliquid polymer which is then cross-linked to provide a final microfiberembedded substrate. Smaller microfibrils are then microimprinted orattached to the top surface of this substrate. Fabrication of alignedmicrofibrils with controlled density and embedding them inside a polymermatrix are not described.

The microfiber fabrication methods described above are very expensivefor producing commercial quantities of adhesive materials. Moreover,they cannot efficiently and controllably produce angled fibers. Adhesionand overall work of adhesion of the microfiber arrays are measured andcompared with the models to observe the effect of fiber geometry andpreload.

Accordingly, there is a need for improved dry adhesives and improvedmethods for making dry adhesives. In particular, there is a need for dryadhesives having greater adhesive forces and improved durability. Inaddition, there is a need for methods of making dry adhesives with lowercosts of production. Those and other advantages of the present inventionwill be described in more detail hereinbelow.

SUMMARY OF THE INVENTION

The present invention provides a method for the fabrication of polymermicrofiber arrays with precisely controlled geometry and density througha micromolding process which duplicates lithographically formed mastertemplate structures with a desired fiber material. This techniqueenables fabrication of fiber array patches inexpensively and with highyields. This is a significant advantage with compared to other proposedfibrillar adhesive fabrication techniques [A. K. Geim, S. V. Dubonos, I.V. Grigorieva, K. S. Novoselov, A. A. Zhukov, and S. Y. Shapoval.Microfabricated adhesive mimicking gecko foot-hair. Nature Materials,2:461-463, 1 Jun. 2003.], [M. T. Northen and K. L. Turner. A batchfabricated biomimetic dry adhesive. Nanotechnology, 16:1159-1166,2005.], [Y. Zhao, T. Tong, L. Delzeit, A. Kashani, M. Meyyappan, and A.Majumdar. Interfacial energy and strength ofmultiwalled-carbon-nanotube-based dry adhesive. Journal of VacuumScience 4 Technology B: Microelectronics and Nanometer Structures,24:331-335, 2006.]. Also, this method allows for design and fabricationof fibers with non-vertical orientation.

The present invention provides methods for fabrication of vertical andangled micro- and nanofibers with adhesive qualities. Polymer micro- andnanofiber arrays are fabricated through a micro molding process whichduplicates lithographically formed master template structures with adesired fiber material. This technique enables fabrication of fiberarrays inexpensively and with high yields, and enables the fabricationof fibers with controlled angles.

Many other variations are possible with the present invention. Forexample, different materials may be used to make the fibers and the dryadhesive, and the geometry and structure of the fibers and the dryadhesive may vary. In addition, different types of etching and othermaterial removal processes, as well as different deposition and otherfabrication processes may also be used. These and other teachings,variations, and advantages of the present invention will become apparentfrom the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings for thepurpose of illustrating the embodiments, and not for purposes oflimiting the invention, wherein:

FIGS. 1 a-1 f illustrate one embodiment of the method according to thepresent invention;

FIGS. 1 g-1 j illustrate another embodiment of the method according tothe present invention;

FIGS. 2 a and 2 b illustrate another embodiment of the method accordingto the present invention;

FIG. 3( a) illustrates the forces on the tip of a single fiber and FIG.3( b) illustrates the fixed-guided boundary conditions of a fiber incontact with a locally flat surface during compression and extension.The angle of the fiber at both ends remains fixed while the tip movesonly in the n direction;

FIG. 4 illustrates a sphere retracting from an array of fibers (not toscale). The fibers close to the middle of the sphere are compressed, andthe ones that are in contact on the edges are stretched. The arrows inthe diagram represent the direction and relative magnitude of the forceson the fibers;

FIGS. 5( a)-5(d) illustrate process steps of the polymer microfiberfabrication according to another embodiment of the present invention. InFIG. 5( a), a thin layer of SU-8 is spun on a glass substrate thenexposed and cured; in FIG. 5( b), a thicker layer of SU-8 is spun whichwill become the fibers; in FIG. 5( c), the thick layer is patterned withUV exposure; and in FIG. 5( d), the SU-8 photoresist is developed,leaving the desired fiber array;

FIG. 6( a) is an SEM micrograph of 6 μm diameter SU-8 polymer highaspect ratio fiber array; and FIG. 6( b) illustrates that as fiberlength increases (from left to right) past the maximum stable length thefibers collapse together;

FIG. 7( a) is an SEM micrograph of 8 μm diameter SU-8 high aspect ratioangled fiber array master template; and FIG. 7( b) illustratespolyurethane molded microfibers with the same geometry;

FIG. 8 illustrates optical microscope profile view image frames from atypical measurement showing 25 μm diameter long angled ST-1060polyurethane fibers with a line overlaid on the edge of the glasshemisphere for clarity, wherein FIG. 8( a) illustrates before contactwith the glass hemisphere, wherein FIG. 8( b) illustrates fibers duringcompression without loss of tip contact, wherein FIG. 8( c) illustratesfiber extension of the three last attached fibers, and wherein FIG. 8(d) illustrates the fibers after contact is lost;

FIG. 9 illustrates sample force-distance experimental data during atypical profile view mode imaging of a manually cut cross-section oflong vertical ST-1060 polyurethane fibers. The images from the timestamped video correspond to the labeled points in the force-distancecurve, and show the compression due to the preload force (point B) andextension during unloading (points C-F) with solid lines overlaid on theedge of the hemisphere and black marks on n=0 for clarity. Maximumseparation force occurs at point E;

FIG. 10 illustrates force-distance plots for the tested flat, long andshort vertical, and long angled fiber array samples given in Table 1.Each line is an average of data over three runs under identicalconditions with eight mN preload;

FIG. 11( a) illustrates maximum separation force as a function ofpreload for the tested samples, and FIG. 11( b) illustrates energydissipation as a function of preload. The simulation results are lines(LV: long vertical, LA: long angled, SV: short vertical) and theexperimental data are markers;

FIG. 12( a) illustrates inverted microscope image frames during testing25 μm diameter long vertical fibers from below; and FIG. 12( b)illustrates real maximum contact area created from the same frame byimage subtraction methods;

FIG. 13( a) illustrates adhesion strength and FIG. 13( b) illustratesoverall work of adhesion as a function of preload pressure. Thesimulation results are lines (LV: long vertical, LA: long angled, SV:short vertical) and the experimental data are markers;

FIG. 14( a) illustrates a scanning electron microscope image of theprofile view of a polyurethane elastomer microfiber array with spatulatetips with around 5 μm fiber stem diameter, 9 μm tip diameter, 20 μmlength, and 44% fiber density;

FIG. 14( b) illustrates experimental pull-off force (adhesion) (F_(ad))data for elastomer microfiber arrays on a silicon disk with 0.43 mmradius with four different normalized backing layer thicknesses (h/a)for a 10 mN preload (four data points were plotted for each backingthickness on different fiber array locations);

FIG. 15 illustrates schematics of the force analysis in two differentbacking layer thickness cases under constant displacement: (a) fibers onan infinitesimal thick backing layer; (b) fibers on a very thick backinglayer;

FIGS. 16 a-16 e illustrate one embodiment of making multi-layer fibersand fiber arrays according to the present invention; and

FIGS. 17 a-17 c illustrate scanning electron microscope images ofmulti-layer fibers and fiber arrays fabricated according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to dry adhesives and methods formaking dry adhesives. The term “dry adhesive”, as used herein, refersgenerally to individual dry adhesive fibers and also to materialsincluding a plurality of dry adhesive fibers connected together. Thecombination of a plurality of fibers connected together, such as with abacking layer, will sometimes also be referred to herein as a fiberarray. The present invention will also be described in terms of micro-and nanofibers, although the present invention is applicable to a widevariety of sizes and is not necessarily limited to a particular sizerange. In addition, particular embodiments of the present invention willoften describe as using “photoresist”. However, photoresist may besubstituted for any energy sensitive material, and energy sensitivematerial means any material that selectively changes its characteristicswhen exposed to energy in order to accomplish the methods described inthe present invention. The energy sensitive material can be positive ornegative. The energy may be visible light, ultraviolet light, x-rays,and other forms of energy. The energy sensitive material used herein maybe either positive or negative, such as positive or negativephotoresist, depending on the particular application.

The application also refers to the following terms, words, and phrasesthat have particular meaning with regards to the present invention. Ageometric feature being micro or microscale means that at least one ofthe characteristic lengths of the feature in any 3D direction should bebetween 0.5-500 micrometers in length. Micropatterned surfaces aresurfaces which have at least one micro scale feature on them. Ageometric feature being nano or nanoscale means that at least one of thecharacteristic lengths of the feature in any 3D direction should bebetween 0.2-500 nanometers in length. Nanopatterned surfaces aresurfaces which have at least one nanoscale feature on them. Micro andnanopatterned surfaces refer to surfaces with any combination andquantity of microscale (0.5-500 micrometers in length) and nanoscale(0.2-500 nanometers in length) features on them. The characteristicdiameters of the micro and nanopatterned features can range from 0.2-500micrometers and 0.2-500 nanometers for microscale and nanoscalefeatures, respectively. Therefore, surfaces of the present invention cancontain only microscale features, only nanoscale features, or bothmicroscale and nanoscale features.

The present invention provides a method for fabrication of polymermicrofiber arrays a through a micro-molding process which duplicateslithographically formed master template structures with a desired fibermaterial. This technique enables fabrication of fiber array patchesinexpensively and with high yields. This is a significant advantage withcompared to other proposed fibrillar adhesive fabrication techniques [A.K. Geim, S. V. Dubonos, I. V. Grigorieva, K. S. Novoselov, A. A. Zhukov,and S. Y. Shapoval. Microfabricated adhesive mimicking gecko foot-hair.Nature Materials, 2:461-463, 1 Jun. 2003.], [M. T. Northen and K. L.Turner. A batch fabricated biomimetic dry adhesive. Nanotechnology,16:1159-1166, 2005.], [Y. Zhao, T. Tong, L. Delzeit, A. Kashani, M.Meyyappan, and A. Majumdar. Interfacial energy and strength ofmultiwalled-carbon-nanotube-based dry adhesive. Journal of VacuumScience 4 Technology B: Microelectronics and Nanometer Structures,24:331-335, 2006.]. Also, this method allows for design and fabricationof fibers with non-vertical orientation.

FIGS. 1 a-1 f illustrate one embodiment of the method according to thepresent invention to make the final product or dry adhesive structure 30shown in FIG. 1 f from a negative mold structure 26 shown in FIG. 1 d.FIG. 1 a illustrates a backing material 10, such as a substrate or acoating, which is provided as the starting point for the method. Thebacking material 10 may be, for example, glass, having a thicknessranging from 1 micrometer to 1 millimeter. Glass offers severaladvantages, such as reducing UV reflections during angled lithography.However, the backing material 10 may also be materials other than glass,e.g. a coating. The coating 10 may be, for example a UV antireflectivecoating, having a thickness ranging from 1 nm to 100 micrometers.

A template backing layer 12 is formed on the backing material 10. Thebacking layer 12 may be formed from photoresist, such as SU-8, or fromother forms of photoresist, other forms of photosensitive polymers, orfrom other materials that have properties that can be selectivelychanged by exposing the material to light or by exposing the material toother energy sources. The backing layer 12 may be further processed soas to provide the desired characteristics. For example, it has beenfound that the template backing layer 12 may be baked and uniformlyexposed to UV energy so as to provide improved adherence between thebacking material 10 and a subsequently applied template layer 14.

It has been discovered that it is often advantageous to use the samephotoresist material on both the backing layer 12 and the template layer14. In particular, the bonding between the backing layer 12 and thestructure formed from the template layer 14 is often better than insituations when different photoresist materials are used for those twolayers 12, 14. However, any other liquid or solid coating material thatcould have a strong bonding with the fiber photoresist polymer could bealso used as the backing material 12, such as anti-reflective oradhesive coatings.

The following is one embodiment of the method of forming the backinglayer 12 of SU-8 photoresist:

baking the backing layer 12 on hotplate at 65 C for one minute;

ramping the temperature of the backing layer to 95 C;

baking the backing layer at 95 C for two minutes;

allowing the backing layer to cool slowly;

exposing the backing layer 12 to UV light for four minutes;

baking the backing layer at 65 C for one minute (this step is sometimesreferred to as “post exposure baking” (“PEB”);

ramping the temperature of the backing layer 12 to 95 C and maintainthat temperature for one minute; and slowly cooling the backing layer12.

Many variations of this method are possible, and this description isillustrative of the present invention, and not limiting. For example,the temperatures and time periods may be changes, and steps may beadded, removed, and their order changed.

The template layer 14 is formed on the template backing layer 12. Thetemplate layer 14 may be formed, for example, from SU-8 photoresist orfrom other materials as described above. The template layer 14 isexposed to a predetermined pattern of energy as is known insemiconductor and other lithography technologies. The energy may beapplied vertically or orthogonally to the surface of the backingmaterial 10, as in the illustrated embodiment, or it may be applied atan angle, or obliquely to the surface of the backing material 10. Insome embodiments a patterned layer 16 may be formed on top of thetemplate layer 14 as part of the process of exposing the template layer14 to a predetermined pattern of energy. The patterned layer 16 may beformed with, for example, photoresist, or the patterned layer 16 may bea mask. The mask may be a contact or non-contact mask, and a positive ora negative mask compatible with the energy sensitivity material. Thepattern of the mask can be any arrangement to form the desired fiberarray pattern of the template structure 20, wherein template structure20 is a positive or same representation of the final product or dryadhesive structure 30 shown in FIG. 1 f. After the template layer 14 isexposed to the predetermined pattern of energy, the patterned layer 16may be removed.

Thereafter, a portion of the template layer 14 is removed. The portionof template material 14 that is removed is related to the predeterminedpattern of energy applied to the template layer 14. The energy may be,for example, ultraviolet light x-rays, or other forms of energy as isknown in semiconductor fabrication and lithography. Depending on whetherthe photoresist of the template layer 14 is positive or negative, theremoved material may have a direct or an inverse relationship to thepattern of energy provided to the template layer 14.

FIG. 1 b illustrates a template structure 20 which remains aftermaterial is removed form the template layer 14. The template structure20 is formed from the remaining photoresist from the template layer 14and is connected to the backing material 10 via the template backinglayer 12. In this embodiment, the template structure 20 may serve as thefinal product of this method. In that case, the template structure 20 isthe dry adhesive structure 30 (described below beginning with FIG. 1 e),and the template layer 14 is the dry adhesive material 28 (describedbelow beginning with FIG. 1 e). However, the template structure 20 isnot suitable for some applications (for example, some materials used tomake the template structure 20 are too brittle for some applications),and in those cases the template structure 20 may be used as a templateto make a mold, which-in turn can be used to make the desired dryadhesive structure. In that case, the template structure 20 correspondsto the structure that is desired, although the template structure 20 isnot the final product. In this embodiment, the final structure will bemade from a material different from that of the template structure 20.

FIG. 1 c illustrate a mold material 22 that has been provided in thetemplate structure 20. The mold material 22 is different from thematerial of the template structure 20, and it may be, for example,silicone rubber or other materials. The mold material 22 may be, forexample, poured into the template structure 20 or otherwise deposited orprovided into the template structure 20.

The mold material 22 is allowed to form a mold structure which iscomplimentary to the template structure 20. This may happen, forexample, by allowing the mold material 22 to cure or to otherwise allowit to take the desired form. In the illustrated embodiment, the moldmaterial is allowed to overflow the template structure 20, forming amold backing layer 24 which connects the various parts of the moldmaterial 22. The mold backing layer 24 may be desired, for example, as abase or platform on which to support the structure formed by the moldmaterial 22. However, the mold backing layer 24 is not required with thepresent invention, and the present invention may be practiced withoutthe mold backing layer 24.

After the mold material 22 has taken the desired form, the mold material22 is separated from the template structure 20.

FIG. 1 d illustrates the negative mold structure 26 which results afterthe mold material 22 has been separated from the template structure 20.The negative mold structure 26 has been rotated 180 degrees in theillustrated embodiment to facilitate the remainder of the method, aswill be described hereinbelow. As can be seen in this figure, the moldbacking layer 24 serves as a base for the negative mold structure 26.

FIG. 1 e illustrates the negative mold structure 26 filled with amaterial 28 that will become the dry adhesive or fiber array fabricatedby this process. The dry adhesive material 28 is allowed to form a dryadhesive structure 30 including one or more fibers 34. The fibers 34include a tip having a flat surface 36 at the tip 38. The dry adhesivestructure 30 is complimentary to the negative mold structure 26. The dryadhesive material 28 may be, for example, polymers, metals, or any otherdesired material. The dry adhesive material 28 forms the dry adhesivestructure 30 by, for example, being allowed to cure or otherwise takinga desired form.

The dry adhesive structure 30 may overfill the negative mold structure26, so as to form a dry adhesive backing layer 32 which connects the dryadhesive material 28 within the recesses of the negative mold structure26. In the illustrated embodiment, the backing layer 32 is outside ofthe negative mold structure 26 and on top of the negative mold structure26. In other embodiments, the negative mold structure 26 may include arecess at the top in which the backing layer 32 may be formed so thatthe backing layer 32 is within the negative mold structure 26. Thethickness of the backing layer 32 may be varied and controlled, so as toform a desired thickness. The thickness of the backing layer 32 can havea significant affect on the performance of the dry adhesive structure30, and this is discussed in more detail hereinbelow. After the dryadhesive material 28 is allowed to form a dry adhesive structure 30, itis removed from the negative mold structure 26.

FIG. 1 f illustrates dry adhesive structure 30 after it is separatedfrom the negative mold structure 26 and rotated 180 degrees, leaving thedry adhesive structure 30 in the desired form. The dry adhesivestructure 30 has the same shape as the template structure 20 illustratedin FIG. 1 b. Furthermore, the negative mold structure 26 may be usedmore than one time in order to form more than one dry adhesive structure30. As a result, the method steps illustrated and described with respectto FIGS. 1 a-1 d may be performed one time, and the method stepsillustrated and described with respect to FIGS. 1 e and 1 f may beperformed many times.

FIGS. 1 g-j illustrate another embodiment of the method according to thepresent invention to make the final product or dry adhesive structure 30shown in FIG. 1 j from a negative mold structure 26 b shown in FIG. 1 h,thereby eliminating the steps shown in FIGS. 1 b and 1 c. Dry adhesivematerial 28 must be of different material than mold layer 14 b such thatdry adhesive structure 30 can be removed from negative mold structure 26b.

The mold layer 14 b is formed on the mold backing layer 12 b. The moldlayer 14 b may be formed, for example, from photoresist or from otherenergy sensitive materials, as described above, that will form apliable, resilient dry adhesive structure capable of withstanding tensor hundreds of molding operations after mold layer 14 b is exposed to apredetermined pattern of energy. The energy may be applied vertically ororthogonally to the surface of the backing material 10, as in theillustrated embodiment, or it may be applied at an angle, or obliquelyto the surface of the backing material 10. In some embodiments apatterned layer 16 b may be formed on top of the mold layer 14 b as partof the process of exposing the mold layer 14 b to a predeterminedpattern of energy. The patterned layer 16 b may be formed with, forexample, photoresist, or the patterned layer 16 b may be a mask. Themask may be a contact or non-contact mask, and a positive or a negativemask compatible with the energy sensitivity material. The pattern of themask can be any arrangement to form the desired fiber array pattern ofthe negative mold structure 26 b, wherein negative mold structure 26 bis a negative or opposite representation of the final product or dryadhesive structure 30 shown in FIG. 1 j. After the mold layer 14 b isexposed to the predetermined pattern of energy, the patterned layer 16 bmay be removed.

Thereafter, a portion of the mold layer 14 b is removed. The portion ofmold layer 14 b that is removed is related to the predetermined patternof energy applied to the mold layer 14 b. The energy may be, forexample, ultraviolet light x-rays, or other forms of energy as is knownin semiconductor fabrication and lithography. Depending on whether thephotoresist of the mold layer 14 b is positive or negative, the removedmaterial may have a direct or an inverse relationship to the pattern ofenergy provided to the mold layer 14 b.

FIG. 1 h illustrates a negative mold structure 26 b which remains aftermaterial is removed form the mold layer 14 b. The negative moldstructure 26 b is formed from the remaining photoresist from the moldlayer 14 b and is connected to the backing material 10 b via the moldbacking layer 12 b. In this embodiment, the negative mold structure 26 bmay serve as the final product of this method.

FIG. 1 i illustrates the negative mold structure 26 b filled with amaterial 28 that will become the dry adhesive or fiber array fabricatedby this process. The dry adhesive material 28 is allowed to form a dryadhesive structure 30 including one or more fibers 34. The fibers 34include a tip having a flat surface 36 at the tip 38 shown in FIG. 1 j.The dry adhesive structure 30 is complimentary to the negative moldstructure 26 b. The dry adhesive material 28 may be, for example,polymers, metals, or any other desired material. The dry adhesivematerial 28 forms the dry adhesive structure 30 by, for example, beingallowed to cure or otherwise taking a desired form.

The dry adhesive structure 30 may overfill the negative mold structure26 b, so as to form a dry adhesive backing layer 32 which connects thedry adhesive material 28 within the recesses of the negative moldstructure 26 b. In the illustrated embodiment, the backing layer 32 isoutside of the negative mold structure 26 b and on top of the moldstructure 26. In other embodiments, the negative mold structure 26 b mayinclude a recess at the top in which the backing layer 32 may be formedso that the backing layer 32 is within the negative mold structure 26 b.The thickness of the backing layer 32 may be varied and controlled, soas to form a desired thickness. The thickness of the backing layer 32can have a significant affect on the performance of the dry adhesivestructure 30, and this is discussed in more detail hereinbelow. Afterthe dry adhesive material 28 is allowed to form a dry adhesive structure30, it is removed from the negative mold structure 26 b.

FIG. 1 j illustrates dry adhesive structure 30 after it is separatedfrom the negative mold structure 26 b and rotated 180 degrees, leavingthe dry adhesive structure 30 in the desired form. The dry adhesivestructure 30 has the negative or opposite shape as the negative moldstructure 26 b illustrated in FIG. 1 h. Furthermore, the negative moldstructure 26 b may be used more than one time in order to form more thanone dry adhesive structure 30. As a result, the method steps illustratedand described with respect to FIGS. 1 g and 1 h may be performed onetime, and the method steps illustrated and described with respect toFIGS. 1 i and 1 j may be performed many times.

FIGS. 2 a and 2 b illustrate another embodiment of the method accordingto the present invention in which the template structure 20 is the dryadhesive structure 30. In that embodiment, the dry adhesive structure 30is formed directly and without the use of the negative mold structure 26or negative mold structure 26 b. FIG. 2 a illustrates a backing material10 which is provided as the starting point for the method. The backingmaterial 10 may be the same as that described above with respect to FIG.1 a.

A dry adhesive backing layer 40 is formed on the backing material 10.The dry adhesive backing layer 40 may be formed in same manner and fromthe same materials as the template backing layer 12. However, ratherthan provide improved adherence between the backing material 10 and amaterial that will become a template for a mold, the backing layer 40provides improved adherence between the backing material 10 and amaterial 42 that will become the dry adhesive structure 30. The backinglayer itself 12/40, however, may be the same in either application.

The dry adhesive layer 42 is an energy sensitive material, and the dryadhesive layer 42 is formed on the dry adhesive backing layer 40.However, unlike the dry adhesive material 28 used with respect to FIGS.1 e and 1 f, a negative mold structure 26 or a negative mold structure26 b are not used this embodiment. Instead, the dry adhesive layer 42 isformed from an energy sensitive material, such as photoresist or theother materials discussed above with regard to the template layer 14.

The dry adhesive layer 42 is exposed to a predetermined pattern ofenergy as described above. The energy may be applied vertically so as toform vertical features, as in the illustrated embodiment, or it may beapplied at an angle so as to form angle features. In some embodiments apatterned layer 16 may be formed on top of the dry adhesive layer 42 aspart of the process of exposing the dry adhesive layer 42 to apredetermined pattern of energy. The patterned layer 16 may be formedwith, for example, photoresist. After the dry adhesive layer 42 isexposed to the predetermined pattern of energy, the patterned layer 16may be removed.

Thereafter, a portion of the dry adhesive layer 42 is removed. Theportion of dry adhesive layer 42 that is removed is related to thepredetermined pattern of energy applied to the dry adhesive layer 42.Depending on whether the photoresist or other energy sensitive materialof the dry adhesive layer 42 is positive or negative, the removedmaterial may have a direct or an inverse relationship to the pattern ofenergy provided to the dry adhesive layer 42.

FIG. 2 b illustrates dry adhesive structure 30 which remains aftermaterial is removed form the dry adhesive layer 42. The dry adhesivestructure 20 is formed from the remaining photoresist from the dryadhesive layer 42 and is connected to the backing material 10 via thedry adhesive backing layer 42.

FIG. 3( a) illustrates the forces on the tip of a single fiber. FIG. 3(b) illustrates the fixed-guided boundary conditions of a fiber incontact with a locally flat surface during compression and extension.The angle of the fiber at both ends remains fixed while the tip movesonly in the n direction. These boundary conditions are critical to modelthe correct single fiber adhesion on a locally flat surface.

FIG. 4 illustrates a sphere retracting from an array of fibers (not toscale). The fibers close to the middle of the sphere are compressed, andthe ones that are in contact on the edges are stretched. The arrows inthe diagram represent the direction and relative magnitude of the forceson the fibers. Modeling the contact of any fiber array 30 with aspherical asperity is important to understand the fiber array's 30macroscale adhesion on a single spherical asperity, which gives ananalogy of adapting to a surface roughness with these fibers.

Master Template Fabrication

The present invention will now be described in terms of specificexamples. These examples are illustrative of the present invention arenot limiting. In these examples, micron scale fibers are selectedbecause they allow consistent fabrication results and they are largeenough to be easily visible with optical microscopy. However, the fiberfabrication techniques of this invention can be directly applied andextended to few micron or nanoscale fibers.

The template structure 20 (also referred to as the “master template”)for the molding process will be described in terms of using SU-8photoresist, although in other embodiments the template structure 20 mayalso use other materials as described herein.

FIGS. 5( a)-5(d) illustrate an embodiment of the method according to thepresent invention that uses SU-8 photoresist (SU-8 2025, MicroChem) 12on a glass wafer substrate 10. The glass wafer substrate 10 is used toprevent UV reflections during angled lithography. First, a thin layer ofdiluted SU-8 photoresist 12 is spun onto a substrate wafer 10 to providea thin polymer backing 12 for the microfibers that will be formed. Thelayer 12 is baked and uniformly exposed with UV light (FIG. 5( a)). Thisbacking layer 12 was found to be important for the process since theadherence between SU-8 and the substrate 10 is too weak to keep the highaspect ratio microfibers anchored firmly otherwise. Next, another layerof SU-8 14 is spun atop the first layer 12 as seen in FIG. 5( b). Thethickness of this layer 14 determines the height of the microfibers,usually between 30 μm and 100 μm A contact mask (not shown) with 2 to 20μm diameter circular clear areas in square packing arrangement is usedto pattern the template layer 14 with directional UV light (FIG. 5( c)).The wafer is then developed in a liquid bath (SU-8 developer, MicroChem)for up to 20 minutes to remove the unexposed photoresist from thetemplate layer 14, leaving the pattern of exposed photoresist pillars orfiber arrays forming the template structure 20 (FIG. 5( d)).

FIGS. 6( a) and 6(b) illustrate scanning electron microscope images offiber arrays 30 formed using the present invention. In this embodimentof the invention, the exposure of the UV light is perpendicular to thesurface of the substrate 10 to form vertically oriented fibers. Resultsfrom this lithographic technique can be seen in FIG. 6( a). Large fiberarrays 30 on the order of 300 mm² of long hair-like independent fibersare produced. Fibers with diameters from 4.μm to 25 μm diameter areproduced with aspect ratios of up to 20:1. 10:1 aspect ratio fiberarrays 30 exhibit excellent uniformity and tend to remain upright.

UV diffraction and the SU-8 properties limit the resolution toapproximately 2 μm for high aspect ratio structures, and consistentfabrication of features of this size is challenging. More advancedexposure techniques may be employed to reach higher aspect ratios orsmaller diameters. For example, Bogdanov et al. have demonstrated usingx-ray lithography that it is possible to form vertical independent SU-8fibers with aspect ratios in excess of 50:1 [A. L. Bogdanov and S. S.Peredkov. Use of su-8 pr for very high aspect ratio x-ray lithography.Microelectronic Engineering, 53:493-496, 2000.]. X-ray lithography alsoallows for sub-micron diameter features [R.-Y. Shew, J.-T. Hung, T.-Y.Huang, K.-P. Liu, and C.-P. Chou. High resolution x-ray micromachiningusing su-8 resist. Journal of Micromechanics and Microengineering,13:708-713, 2003.] which can be used for nanofiber fabrication.

To avoid fiber clumping, the geometry parameters for the fibers areselected by considering energy balance equation given by [K. L. Johnson,K. Kendall, and A. D. Roberts. Surface energy and contact of elasticsolids. Proceedings of the Royal Society of London. Series A,324:301-313, 1971.]. The lithography mask 16 determines the radius andspacing between fibers, leaving only fiber length and angle asvariables. Fiber length is determined by the thickness of the SU-8 layer14 and can be varied by changing the SU-8 viscosity or spin speed.

FIG. 6( b) illustrates the dependence of fiber collapse on fiber length.Collapse occurs during the drying phase after development and rinsingwhen water droplets repel the hydrophobic fibers pressing them intocontact with each other. The dependence of collapse on fiber length canbe seen in FIG. 6( b) where the only variation across the image isincreasing fiber length from left to right. The shorter fibers on theleft side are standing independently alongside the longer fibers whichhave collapsed laterally. Collapse generally occurs between more than apair of fibers, so in addition to the distance between adjacent fibers,the arrangement pattern of the fibers determine clumping conditions.Square packing of fibers has been determined to allow close packingwithout clumping [C. Majidi, R. Groff, and R. Fearing. Clumping andpacking of hair arrays manufactured by nanocasting. Proc. of the ASMEInternational Mechanical Engineering Congress and Exposition, 579-584,2004.].

FIG. 7( a) illustrates angled microfibers that were fabricated using thesame lithographic technique as described above. By varying the UVexposure angle by tilting the wafer during exposure, the fibers areformed at a non-perpendicular angle to the substrate 10 surface. UsingSnell's law, the UV exposure angle can be calculated for the desiredfiber angle [K.-Y. Hung, H.-T. Hu, and F.-G. Tseng. Application of 3dglycerol-compensated inclined-exposure technology to an integratedoptical pick-up head. Journal of Micromechanics and Microengineering,14:975-983, 2004.]. From our tests, tilting the wafer with around45.degree. slope reliably formed fibers with angles of around 25.degree.from vertical. Hung et al. showed that angled structures of up to60.degree. from vertical are achievable by immersing the wafer inglycerol during exposure to decrease refraction [K.-Y. Hung, H.-T. Hu,and F.-G. Tseng. Application of 3d glycerol-compensatedinclined-exposure technology to an integrated optical pick-up head.Journal of Micromechanics and Microengineering, 14:975-983, 2004.]. Oneadvantage of this method for fabricating angled fibers is that the tipshape is flat and roughly parallel to the substrate 10 rather thanperpendicular to the fiber axis. This increases the contact area of thetips when in contact with a surface.

Micromolding

Since the SU-8 material properties are not ideal for use as a structuralmaterial due to its brittleness and weak bond with the backingsubstrate, it is desirable to create a mold in which to replicate thefibers with a different polymer material. Moreover, molding enables theselection of wide range of polymer materials as the fiber material, andthe master template can be used tens or hundreds of times whichincreases the fabrication speed and reduces the cost significantly. Acompliant mold is fabricated by pouring a liquid silicone rubber (HS-II,Dow Corning) over the wafer and allowing it to cure at room temperaturefor 24 hours. Once cured, the mold is carefully peeled away from thetemplate wafer resulting in a flexible mold with the negative shape ofthe SU-8 fibers. This mold is used to vacuum mold liquid polyurethanesor other curable materials (except silicone rubbers which bond to thetemplate rubber material) with the desired physical properties. Oncecured and de-molded, the polyurethane fibers have roughly the samegeometry as the original SU-8 fibers.

Using this method, the microfiber material may be altered to suit theintended design. However, the same lateral collapse laws apply, so thedesign must stay within the lateral collapse requirements for both SU-8(to create the template) and the secondary fiber material to ensure thatself-supporting independent fibers are formed. This technique has beensuccessfully implemented to create angled high aspect ratio polyurethanefibers from SU-8 templates 20, as shown in FIG. 7. However, thesuccessful de-molding of the high aspect ratio SU-8 fiber arrays 30without fracturing many of the fibers is a significant challenge.

Macroscale Adhesion Experiments

Fiber array 30 test samples were fabricated using the methods describedabove with polyurethane elastomers (ST-1087 and ST-1060; BJBEnterprises) which have high tensile strength (6 MPa) and are availablecommercially with wide range of elastic moduli. The hardness of thesepolymers are Shore 83 A and 60 A, respectively and were determinedthrough tensile testing to have Young's modulus of approximately 9.8 MPaand 2.9 MPa, respectively. Fiber arrays 30 with varied lengths andangles were fabricated for testing as described in Table 1.

TABLE 1 Diameter Length Angle Fiber Sample Type Height (2a) (L) (θ)Spacing (d) Short 48 μm 20 μm 48 μm 0° 40 μm Vertical Long 100 μm  25 μm100 μm  0° 40 μm Vertical Long Angled  75 .μm 25 μm 79 μm 18°  40 μm

These geometries were selected because they allow consistent fabricationresults and are large enough to be easily visible with opticalmicroscopy. The backing layer 12 is approximately 2.5 mm thick for allsamples.

First, an atomic force microscope (AFM) (Veeco CP-II) is used tocharacterize the polyurethane-glass interfacial effective work ofadhesion W_(ƒ) which is used to calculate the adherence of a singlefiber in P_(ƒ)=√(6πα³Kw_(ƒ). In contrast to the data from macroscalemeasurements of a flat polyurethane sample, the AFM data exhibit reducedbulk viscoelastic effects and more closely approximate the single fiberadhesion. In the absence of bulk viscoelastic effects, the surfaceviscoelastic losses and the thermodynamic work of adhesion can becombined into a single term w_(ƒ) which is used in elastic theoryequations [K. R. Shull. Contact mechanics and the adhesion of softsolids. Materials Science and Engineering R, 36:1-45, 2002.] to predictthe adhesion of a single fiber. A 12 μm diameter silica particleattached AFM probe (Novascan) with 14 N/m bending stiffness is used tomeasure the pull-off force between the particle and a flat polyurethanesurface using ten measurements. Using the Johnson-Kendall-Robert (JKR)adhesion model for a sphere-plane interaction, w_(ƒ) is computed from[K. L. Johnson, K. Kendall, and A. D. Roberts. Surface energy andcontact of elastic solids. Proceedings of the Royal Society of London.Series A, 324:301-313, 1971.]

P _(cs)=(3/2)πR _(p) w _(ƒ)  (1)

where P_(CS) is the pull-off force and R_(p), is the particle radius.Using the measured pull-off force and (1), the effective work ofadhesion w_(ƒ) for glass and ST-1060 polyurethane interface was computedas 93 mJ/m².

In most of the previous works [A. K. Geim, S. V. Dubonos, I. V.Grigorieva, K. S. Novoselov, A. A. Zhukov, and S. Y. Shapoval.Microfabricated adhesive mimicking gecko foot-hair. Nature Materials,2:461-463, 1 Jun. 2003.], [B. Yurdumakan, N. R. Raravikar, P. M.Ajayanb, and A. Dhinojwala. Synthetic gecko foot-hairs from multiwalledcarbon nanotubes. Chemical Communications, page 3799-3801, 2005.],micro/nanoscale local adhesion of synthetic fibers was tested using anAFM. Although these local measurements can give detailed insight intothe individual fiber adhesion micro/nanomechanics, the overallmacroscale behavior of the fiber array can be significantly different.Moreover, AFM based or other [Y. Zhao, T. Tong, L. Delzeit, A. Kashani,M. Meyyappan, and A. Majumdar. Interfacial energy and strength ofmultiwalled-carbon-nanotube-based dry adhesive. Journal of VacuumScience 4 Technology B: Microelectronics and Nanometer Structures,24:331-335, 2006.], [M. T. Northen and K. L. Turner. A batch fabricatedbiomimetic dry adhesive. Nanotechnology, 16:1159-1166, 2005.] localmeasurements do not allow observation of the fiber tip contact duringthe adhesion measurements. Therefore, macroscale adhesion and overallwork of adhesion of fabricated polymer microfiber arrays arecharacterized in this work using a custom tensile adhesion measurementsetup with an optical imaging capability to observe the real-timecontact area.

The custom macroscale adhesion measurement system consists of a top-viewreflection type optical microscope (Nikon Eclipse L200) or an invertedoptical microscope (Nikon Eclipse TE200) with an automated highprecision stage (MFA-CC; Newport) which holds a high resolution loadcell (GSO-25; Transducer Techniques Inc.). A 12 mm diameter glasshemisphere (QU-HS-12; ISP Optics) is connected to the load cell. Theadhesive samples are placed on the microscope stage with the fiberarrays facing toward the glass hemisphere. Custom real-time softwarecontrols the stage to move the hemisphere into contact with the fibersample at a fixed velocity until a pre-specified preload force isreached. The hemisphere is then retracted at a speed of 1 j·cm/s untilit detaches from the sample. The software continually captures the forcedata from the load cell as well as time stamped video from themicroscope.

Two types of experiments are possible with this setup. The first mode,referred to as profile view mode, uses a cut cross section of the fiberarray area which contacts only half of the hemisphere under the top-viewmicroscope. This allows the camera to record images and video of thefiber deformation during contact and retraction. Using this setup modeit is possible to observe the compression, bending, buckling, andstretching behavior of the fibers 34 during the testing as seen in FIG.8. In the other mode, inverted view mode, the hemisphere contacts thecenter of the array 30 of fibers. This mode provides more consistentadhesion data and allows viewing of the real contact area. Thus, profileview mode is used to examine the qualitative behavior of the surfacesduring contact, whereas inverted view mode is used for quantitativeanalysis.

For computing the overall work of adhesion for the macroscalemeasurements on the fiber arrays 30, force-distance data from thehemisphere tests are used to calculate the energy dissipation U_(d) bynumerically integrating the area between the loading and unloading forcecurves. The known deflection of the load cell stem is used to correctthe displacement data before the area integration is realized. Correctedforce-distance data from a single measurement is shown in FIG. 9. Inthis case, an array 30 of long vertical polyurethane fibers was testedwith a 1 mm diameter steel sphere. The sphere starts above the fiberarray tips (A). The force during the approach is zero until the spheremakes initial contact with the fibers at n=0. The preload force (P_(p))is the maximum positive force peak (B) when the sphere has penetratedthe fiber array by distance Δ_(p). The force equilibrium point (C)occurs when the tension in the outer fibers is balanced by thecompression of the inner fibers. Note from point (C) and thecorresponding image indicate that this occurs before the sphere iscompletely retracted beyond the fiber tips. When Δ=0 (D), all of thecontacting fibers are in tension. As the sphere continues to retract,the fibers extend and the most highly extended fibers lose contact. Themaximum separation force (P_(a)) is the value of the negative peak (E)where the sum of the forces of the contacting fibers is the greatest.After this point, the total force decreases because the loss in adhesiondue to fibers detaching is greater than the force due to the increasedextension of the attached fibers. Finally, the last fiber is stretched(F) to its maximum (δ_(c)=Δ_(e)) and then pulls off, returning theoverall force to zero.

Experimental Results

The initial results from experiments in profile view mode indicated thatthe stiffer ST-1087 polyurethane fibers lose tip contact whencompressed, greatly reducing the contact area and adhesion. This wasparticularly problematic for angled fibers, which showed almost noextension or adhesion. The more compliant ST-1060 polyurethane fibersdid not exhibit this behavior and remained in full contact whencompressed as seen in FIG. 8( b). FIG. 6( b) also suggests that thefixed-guided boundary conditions used in the modeling section areappropriate. This resulted in the increased extension (FIG. 8( c)) andadhesion of the softer fibers. Therefore, the remainder of experimentsdiscussed in this paper were conducted on the more compliant ST-1060polyurethane fibers.

A series of experiments were conducted in inverted view mode on a flatpolyurethane control sample and the fiber arrays detailed in Table 1.The preload force was varied from 1 to 50 mN, and the approach andretraction speeds were 1 μm/s. Five measurements were taken for eachsample at each of eight preselected preload values, and the contactpoint was moved to a different location on the fiber array for eachmeasurement.

Averaged force-distance data from the experiments at 8 mN preload forall of the sample types are plotted together in FIG. 10, which providesa visual comparison of the performance of the various samples. The flatsample shows the highest stiffness with least penetration depth (Δ_(p))followed by the short vertical fibers, long vertical, and long angledfibers, respectively. The maximum separation force (P_(a)) is higher forthe flat material than for any of the fiber samples. The flat sample hasthe smallest maximum extension (Δ_(c)), followed by the short vertical,long angled, and long vertical fiber samples, respectively.

To examine the effect of preload on maximum separation force, a plot isgenerated from the maximum separation forces for each sample for therange of preload values (FIG. 11( a)). The data are plotted along withthe simulation results for the same fiber and hemisphere geometry. Asimilar plot is generated to examine the effect of preload on dissipatedenergy (FIG. 11( b)). In the figure, the simulation results are thelines (LV: long vertical, LA: long angled, SV: short vertical) and theexperimental data are the markers. All curves show an increase inadhesion with increased preload.

Although the preload values were controlled for each experiment, thecontact areas are not consistent across the various samples because oftheir varying softness. In order to determine adhesion strength andoverall work of adhesion trends, it is necessary to divide the totalforce or dissipated energy by the maximum contact area.

The relationship between preload force and real maximum contact area(A_(max)) was found using image processing methods. Contact areas werecalculated for the same eight preload values for each sample using imagesubtraction methods on the corresponding video. To see A_(max) in agiven frame, a reference frame with no contact is subtracted leavingonly the difference between the two images which are the contactedfibers (FIG. 12). The contact area is taken as the area of the smallestcircle which contains all of the contacting fibers. These A_(max) valuesare used to calculate the adhesion strengths and overall work ofadhesion (W_(t)). For the simulation data, the simulated maximum contactarea is used. Preload pressure, adhesion strength, and work of adhesionare calculated by dividing the preload, maximum separation force, andenergy dissipation (U_(d)) by A_(max), respectively. The adhesionstrength (p_(a)) and overall work of adhesion (W_(t)) values are plottedwith respect to preload pressure in FIG. 13.

The feasibility of fabricating high density and high aspect ratiomicrofiber arrays in large areas has been confirmed. The use of UVphotolithography as a fabrication process for high aspect ratioself-supporting microfibers has been demonstrated, including fabricationof angled fiber arrays. Micromolding high aspect ratio angled polymermicrofiber arrays by means of a compliant intermediate mold wasdemonstrated. Fabricated microfiber array samples were tested using acustom adhesion measurement system.

The Effect of the Backing Layer Thickness.

It has also been found that the thickness of the backing layer 32 has agreater effect on the performance of dry adhesives than was previouslyknown. The effect of the backing layer 32 thickness on adhesion wasinvestigated for single-level elastomer fibrillar adhesives 30. Themethod used to make the fibers used to study the effect of the backinglayer 32 is different than the method of the present invention. However,the inventors believe that the effect of the thickness of the backinglayer 32 is also applicable to the present invention, and the results ofthe study are presented below.

Polyurethane microfiber arrays 30 with spatulated tips 38 on a 160 μmthick backing layer 32 show nine times greater adhesion strength (around22 N/cm²) than those with a 1120 μm thick backing 32. A theoreticalmodel is proposed to explain this difference in which very thin backinglayers 32 promote equal load sharing, maximizing adhesion, while verythick backings can lead to reduced adhesion due to edge stressconcentration. Therefore, backing layer 32 thickness should beconsidered as a significant parameter for design of high performancefibrillar adhesives.

The adhesion of biologically inspired fibrillar dry adhesive has beenstudied extensively in combination with developments of variousfabrication methods. Based on dominant forces of van der Waals [K.Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg,T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full, PNAS, 99,12252 (2002)] and possibly capillary [G. Huber, H. Mantz, R. Spolenak,K. Mecke, K. Jacobs, S. N. Grob, and E. Artz, PNAS, 102(45), 16293(2005)] forces, vertical cylindrical micro/nanofiber arrays [A. K. Geim,S. V. Dubnos, I. V. Grigorieva, K. S. Novoselov, A. A. Zhukov, and S. Y.Shapoval, Nature Materials, 2, 461 (2003)] were proposed as fibrillaradhesives at first. Design parameters for these fibers were proposed asthe fiber radius, aspect ratio [C. Greiner, A. del Compo, and E. Arzt,Langmuir, 23, 3495 (2007)], tip shape [H. Gao and H. Yao, PNAS, 101,7851 (2004)], and material properties [K. Autumn, C. Majidi, R. E.Groff, A. Dittmore, and R. Fearing, J. Exp. Biol., 209, 3558 (2006)].Inspired by footpads of various animals in nature such as insects andgeckos, spatulated tips on single-level cylindrical [S. Kim and M.Sitti, Applied Physics Letters, 89, 261922 (2006)][N. J. Glassmaker, A.Jagota, C-Y. Hui, 86 J. Kim, J. R. Soc. Interface, 1, 22 (2004)], angled[B. Aksak, M. P. Murphy, and M. Sitti, Langmuir, 23, 3322 (2007)] andhierarchical [N. J. Glassmaker, A. Jagota, C-Y. Hui, W. L. Noderer, M.K. Chaudhury, PNAS, 104, 10786 (2007)][A. del Campo and E. Arzt,Molecular Bioscience, 7(2), 118 (2007)] fibers were introduced fordeveloping high performance fibrillar adhesives. In addition, one of therecent findings demonstrates that the real contact perimeter is a moreimportant geometrical factor governing adhesion than the real contactarea [M. Varenberg, A. Peressadko, S. Gorb, and E. Arzt, Applied PhysicsLetters, 89, 121905 (2006)]. However, the role of backing layer 32thickness on adhesion has not been investigated in detail so far.

The backing layer 32 thickness effect on adhesion of elastomericsingle-level microfiber structures 30 will now be described. Although athick backing layer 32 improves the roughness adaptation and fiber 34contact abilities due to increased effective compliance, this studyshows that a thick backing layer 32 could reduce the macroscale adhesionof the fibers 34 on smooth surfaces significantly.

We measured the pull-off force of single-level elastomer fiber array 30samples with different backing layer 32 thicknesses and developed atheoretical model to explain the observed results. Polyurethane(ST-1060, BJB Enterprise) fiber array 30 samples with spatulated tips 38are fabricated using the procedure reported in S. Kim and M. Sitti,Applied Physics Letters, 89, 261922 (2006). Briefly, we first fabricatenegative silicon fiber array templates using deep reactive ion etching.Liquid polyurethane 28 is filled into these silicon negative templates26 and cured. The silicon templates 26 are then etched using XeF2, andthe fibers 34 are released. The final backing layer 32 thickness of eachsample is determined by regulating the gap between the negative template26 and a glass slide on it.

All fiber arrays 30 in our samples have a stem diameter of around fiveμm and a tip and base support diameter of nine μm. The total length of afiber 34 is 20 μm and the minimum spacing between fiber centers is 12 μmas displayed in FIG. 13( a). A custom tensile setup was built to measurethe adhesion of our samples. Our measure of adhesion in this work is thepull-off load. A silicon disk with 0.43 mm radius and nanometer scalesurface roughness was fabricated by patterning a polished silicon (100)wafer using optical lithography and deep reactive ion etching. Thesilicon disk was attached to a load cell (Transducer Techniques,GSO-25), and moved vertically by a motorized stage (Newport, MFA-CC)with 100 nm resolution. The disk was contacted to and retracted from thefiber array sample with specified preload forces and a slow speed (1μm/s) to minimize viscoelastic effects. The maximum pull-off force wasrecorded. In addition, the contact area and the deformation of fiberarray 32 during loading and unloading was recorded using a camera(Dage-MT1, DC330) attached to an inverted optical microscope (Nikon,Eclipse TE200).

Adhesion of four samples with 160, 280, 630, and 1120 μm backing layer32 thicknesses was measured and is shown in FIG. 13( b). Pull-off forceswere measured at five different locations on each fiber array 30 for apreload of ten mN. The sample with the thinnest backing layer 32 (160μm) showed the highest adhesion, with average pull-off force about ninetimes greater than that of the 1120 μm layer 32.

Our interpretation of this surprising finding, that reduced complianceenhances adhesion, lies in the idea that a thinner backing layer 32promotes equal sharing of the load by the fibers 10. As shownschematically in FIG. 14( a), if a displacement 6 were applied to threefibers 10 on an infinitesimally thin backing layer 32, each one wouldexperience the same vertical tension, F_(y1). If the fiber pulled-off ata characteristic force F_(ƒ)=k_(ƒ)δ_(ƒ) where k_(ƒ) is the stiffness ofthe fiber 34 and δ_(ƒ) is the fiber 34 elongation at pull-off, then thepull-off force of the system would be ³F_(ƒ). If the thin layer 32 werereplaced by a very thick backing 32 (FIG. 14( b)), the y-directionforces would be F_(y1)=k_(ƒ)δ and F_(y2)=k_(ƒ)δ+F_(B) where F_(B) is theadditional force required to keep the tip 38 attached to the disk due tonon-uniform deformation of the backing layer 32. Because the backinglayer 32 is elastic, F_(B)=αk_(ƒ)δ for some positive α. The two fibers34 on the side will pull-off when k_(ƒ)δ+F_(B)=F_(ƒ) and the total forceat pull-off would be (3−α)F_(ƒ).

To quantify this idea for a large number of fibers 34 in contact, wenote that the spacing of the fibers 34 are typically very small incomparison with the contact radius a and the thickness of the elasticlayer h. Hence, we can treat these fibers 34 as a foundation consistingof elastic springs between the rigid indenter and the backing layer 32.The foundation can support a normal stress σ, which is related to thedisplacement of foundation, d by σ=kd where k is the stiffness of thefoundation. Note that d is the difference in normal displacement betweenthe surface of the indenter and the backing layer 32. The stiffness canbe determined by assuming that the fibers 34 are bars with height L andeffective cross-sectional area A_(eff), k=ρEA_(eff)/L where ρ is thenumber of fibrils per unit area. From the known geometry and stiffnessof our fibers, k=2.37×10⁻¹⁰ N/m³ where ρ=1/(12×10⁻⁶)² fibers/m², E=3MPa, A_(eff)=πr², r=2.5 μm, and L=14 μm.

The maximum pull-off force occurs in the equal load sharing (ELS)regime, where all the fibers in adhesive contact with the indenter bearthe same load. To see how ELS depends on the backing layer thickness andthe contact area, assume that all the fibers in contact are in thisregime, so at pull-off, we have

σ_(f) =kδ _(ƒ)  (2)

In the ELS limit, the maximum pull-off force F_(max) is directlyproportional to the contact area,

F _(max)=πα²σ_(ƒ)  (3)

where a is the radius of the disk. The ELS limit is strictly valid ifthe backing layer thickness h is very small compared to a. Another limitis a very thick backing layer 32 with very stiff fibers 34, that is,when h/a→∞ and α≡ka/2G is very large where G is the shear modulus. Inthis limit, the interfacial displacement is dominated by the deformationof the elastic layer and the stress distribution is given by theclassical solution of a rigid punch in contact with a half space [K. L.Johnson, Contact Mechanics, Cambridge University Press (1985)]. Thenormal stress at the punch edge has a square root singularitycharacteristic of an opening crack. For α>>1 and h/a>>1, the pull-offforce F_(ad) in this limit can be derived as

F _(ad)=(4F _(max))/(2πα)^(0.5)  (4)

This equation shows that, given F_(max), the maximum extent of strengthreduction can be predicted. The data in FIG. 13( b) show a decrease inpull-off force with increasing thickness. Theoretically, in the limit ofvery small thickness, pull-off force should asymptotically approach theELS limit, which depends on the unknown fiber pull off stress, σ_(ƒ). Inthis limit, the pull-off load is significantly affected by variabilityin σ_(ƒ). This is reflected in the scatter of the pull-off force dataassociated with the sample with the smallest h/a.

The theoretical problem of determining pull-off forces as a function ofα and h/a is more involved and will be addressed in a future work. Insummary, polyurethane fiber arrays 30 with spatulated tips 38 on 160 μmthick backing layer 32 show adhesion strength (around 22 N/cm²), ninetimes greater than fiber arrays 30 with thickness of 1120 μm. Atheoretical model is proposed to explain this difference in which verythin backing layers 32 promote equal load sharing, maximizing adhesion.In the other extreme, very thick backings 32 can lead to reducedadhesion, because of edge stress concentration similar to a rigid punchin adhesive contact with a half space. This work shows the significanceof backing layer 32 thickness on equal load sharing of single-levelfiber arrays 30 on smooth surfaces. The same thickness effect isexpected to happen for micro/nanoscale rough surfaces also.

Although the present invention has generally been described in terms ofmaking several fibers 34 attached to a backing layer 32, the presentinvention may also be used to make a single fiber 10, or to make aplurality of fibers 34 that are not connected to a backing layer 32.Furthermore, the present invention may also include two or more layersof fibers 34 or fiber arrays 30 having two or more layers of fibers 34.One embodiment will be described below.

FIGS. 16 a-16 e illustrate one embodiment of making multi-layer fibersand fiber arrays according to the present invention. Although thisembodiment will be described as using SU-8 photoresist, otherembodiments of the present invention may use other kinds of photoresistas well as materials other than photoresist.

FIG. 16 a illustrates a structure similar to that described withreference to FIG. 1 a. A thin layer of SU8 photoresist is spun onto abacking material 10 and is processed to form a backing layer 12. After apost-exposure-bake, another layer of SU8 photoresist 14/42 is spun abovethe first layer 12.

FIG. 16 b illustrates the additional layer of SU8 photoresist 14/42 isbaked and then exposed to ultraviolet or other energy via a circulararray pattern 16 to form the base fibers 34. These fibers 34 can bevertical or angled. Exposing the wafer to the ultraviolet or otherenergy at a non-orthogonal angle creates the angled fibers 34. Again thewafer is heated to cure the layer.

FIG. 16 c illustrates a last thin layer 50 of SU-8 is spun on top of thefirst two layers 12, 14. This last layer 50 will become the second layerof fibers.

FIG. 16 d illustrates the processing of the material 50 that will becomethe second layer of fibers. In particular, in this embodiment the toplayer 50 is heated and then exposed through a mask 52 with smallerfeatures than the previous exposure, forming smaller fibers 54 atop thebase fibers 34. This step can be done by aligning the masks 16, 52, orby using a uniform mask since the second layer of fibers 54 are thinnerthan the first layer of fibers 34, and will not form all the way down tothe surface, and will be developed away everywhere except for at thetops of the base fibers 34.

FIG. 16 e illustrates the structure after being placed in the developingsolution which removes the unexposed (or exposed but unanchored)material, resulting in the desired hierarchical structures. Inparticular, a second dry adhesive structure 56 is formed on top of thefirst dry adhesive structure 30. The second layer dry adhesive structure56 may be formed on the top surfaces 36 of the fibers 34 in the firstlayer dry adhesive structure 30, or the second layer dry adhesivestructure 56 may be formed on the sides of the fibers 34 in the firstlayer dry adhesive structure 30.

FIGS. 17 a-17 c illustrate scanning electron microscope images ofmulti-layer fibers 34 and fiber arrays 30 fabricated according to thepresent invention.

FIG. 17 a illustrates thinner second layer fibers 54 formed on the tops(or near the tops) of the thicker, first layer fibers 34. In thisembodiment, most of the first layer fibers 34 contain only one secondlayer fiber 54, although the present invention may be used to make firstlayer fibers 34 having many second layer fibers 54.

FIG. 17 b illustrates an array of first layer fibers 34 having no secondlayer fibers 54, and an array of second layer fibers having many secondlayer fibers 54.

FIG. 17 c illustrates second layer fibers 54 clumping together.

Although the present invention has been described in terms of a twolayer fiber array 30, the present invention may also be used to producea fiber array 30 having more than two layers. In addition, the presentinvention is not limited to multilayer fiber arrays 30, and it may alsobe used, for example, to make multilayer individual fibers 34 and tomake other structures.

Although the present invention has generally been described in generalterms and in terms of specific embodiments and implementations, thepresent invention is applicable to other methods, apparatuses, systems,and technologies. The examples provided herein are illustrative and notlimiting, and other variations and modifications of the presentinvention are contemplated, such as any other thick or thin photoresistcan be used as the photosensitive polymer to form fibers, X-ray can beused instead of ultraviolet light to pattern the photoresist polymer toform higher angled (0-80 degrees) and smaller diameter fibers (down totens of nanometer scale), the similar angled fibers could be used asdirectional friction materials where the friction is higher in onedirection than the other direction (this directional friction propertyis important for assembly line treads, climbing robots and tires type ofapplications), and the similar angled fibers could be used as a coatingwhere droplet(s) could be moved on these structures in a directional wayfor liquid droplet transfer. Other variations are also possible. Forexample, although the present invention was generally described in termsof fibers having conical shapes illustrated with circularcross-sections, fibers having other shapes may also be made with thepresent invention. Those and other variations and modifications of thepresent invention are possible and contemplated, and it is intended thatthe foregoing specification and the following claims cover suchmodifications and variations.

1. A method of forming a template structure, comprising the stepsperformed in the following sequence: forming a template backing layer ofenergy sensitive material on a backing material; exposing the templatebacking layer to a predetermined pattern of energy from an energysource; forming a template layer of energy sensitive material on thetemplate backing layer; applying a mask to the template layer; exposingthe template layer to an energy source; and removing the mask and aportion of the template layer not exposed to the energy source to createa pattern of exposed energy sensitive material fiber arrays forming thetemplate structure, wherein the pattern of exposed energy sensitivematerial fiber arrays is selected from the group consisting of (i)polymer micro-scale fibers, (ii) polymer nano-scale fibers, and (iii)polymer micro- and nanoscale fibers.
 2. The method according to claim 1,wherein the step of exposing the template layer to the energy sourcefurther comprises directing the energy source at a perpendicular angleto a surface of the template backing, wherein tip surfaces of theexposed energy sensitive material fiber arrays are flat andsubstantially parallel with the surface of the template backing layer.3. The method according to claim 1, wherein the step of exposing thetemplate layer to the energy source further comprises directing theenergy source at a non-perpendicular angle to a surface of the templatebacking layer, wherein tip surfaces of the exposed energy sensitivematerial fiber arrays are flat and substantially parallel with thesurface of the template backing layer.
 4. A method of forming a negativemold product having polymer micro- and/or nano-fiber arrays, comprisingthe steps performed in the following sequence: providing a templatestructure formed from energy sensitive material product having fiberarrays selected from the group consisting of (i) polymer micro-scalefibers, (ii) polymer nano-scale fibers, and (iii) polymer micro- andnanoscale fibers; depositing a mold material into the templatestructure, wherein the mold material is different from a material of thetemplate structure; allowing the mold material to cure; and separatingthe mold material from the template structure to form the negative moldproduct having polymer micro- and nano-fiber arrays.
 5. The methodaccording to claim 4, wherein the fiber arrays of the template structureare oriented at a perpendicular angle to a surface of a template backinglayer of the template structure, wherein tip surfaces of the fiberarrays are flat and substantially parallel with the surface of thetemplate backing layer.
 6. The method according to claim 4, wherein thefiber arrays of the template structure are oriented at anon-perpendicular angle to a surface of a template backing layer of thetemplate structure, wherein tip surfaces of the fiber arrays are flatand substantially parallel with the surface of the template backinglayer.
 7. A method of forming a product having polymer micro- and/ornano-fiber arrays, comprising the step performed in the followingsequence: providing a negative mold with negative shapes of polymermicro- and/or nano-fiber arrays; depositing a dry adhesive materialhaving one or more polymer micro- and/or nano-fibers into the negativemold; allowing the dry adhesive material to cure; and separating the dryadhesive material from the negative mold to form the product havingpolymer micro- and/or nano-fiber arrays.
 8. The method according toclaim 7, wherein the step of depositing further comprises overfillingthe negative mold to form an integral dry adhesive backing layer.
 9. Themethod according to claim 7, wherein the negative shapes of polymermicro- and/or nano-fiber arrays are oriented at a perpendicular angle toa surface of a template backing layer of the template structure.
 10. Themethod according to claim 7, wherein the negative shapes of polymermicro- and/or nano-fiber arrays are oriented at a non-perpendicularangle to a surface of a template backing layer of the templatestructure.
 11. The method according to claim 8, wherein tip surfaces ofthe polymer micro- and/or nano-fiber arrays of the product are flat andsubstantially parallel with a surface of the integral dry adhesivebacking layer.
 12. A method of forming a product having hierarchicalstructures of polymer micro- and/or nano-fiber arrays, comprising thestep performed in the following sequence: forming a first layer ofenergy sensitive material on a backing material to form a first wafer,wherein the energy sensitive material includes polymer micro- and/ornano-fibers; baking the first wafer; exposing the first layer to apredetermined pattern of energy from an energy source; forming a secondlayer of energy sensitive material on to the first layer to form asecond wafer, wherein the second layer is a first layer of fibers,wherein the energy sensitive material includes polymer micro- and/ornano-fibers; baking the second wafer; exposing the second layer to afirst circular array pattern of energy from an energy source energysource to form base fibers; baking the second wafer to cure the secondlayer; forming a third layer of energy sensitive material on to thesecond layer to form a third wafer, wherein the third layer is a secondlayer of fibers; heating the third layer; exposing the third layer to asecond circular array pattern of energy from the energy source to form asecond layer of fibers, wherein the second layer of fibers being smallerthan the base fibers; and removing the first and second layers notexposed to the first and second circular array patterns of energy toform the product having hierarchical structures of polymer micro- and/ornano-fiber arrays.
 13. The method according to claim 12, wherein thestep of exposing the first layer to an energy source further comprisesdirecting the energy source at a perpendicular angle to a surface of thefirst wafer to form polymer micro- and/or nano-fiber arrays at theperpendicular angle to the surface of the first wafer.
 14. The methodaccording to claim 12, wherein the step of exposing the first layer toan energy source further comprises directing the energy source at anon-perpendicular angle to a surface of the first wafer to form polymermicro- and/or nano-fiber arrays at the non-perpendicular angle to thesurface of the first wafer.
 15. The method according to claim 12,wherein the step of exposing the second layer to an energy sourcefurther comprises directing the energy source at a perpendicular angleto a surface of the second wafer to form polymer micro- and/ornano-fiber arrays at the perpendicular angle to the surface of thesecond wafer.
 16. The method according to claim 12, wherein the step ofexposing the second layer to an energy source further comprisesdirecting the energy source at a non-perpendicular angle to a surface ofthe second wafer to form polymer micro- and/or nano-fiber arrays at thenon-perpendicular angle to the surface of the second wafer.
 17. Themethod according to claim 12, wherein the step of exposing the thirdlayer to an energy source further comprises directing the energy sourceat a perpendicular angle to a surface of the third wafer to form polymermicro- and/or nano-fiber arrays at the perpendicular angle to thesurface of the third wafer.
 18. The method according to claim 12,wherein the step of exposing the third layer to an energy source furthercomprises directing the energy source at a non-perpendicular angle to asurface of the third wafer to form polymer micro- and/or nano-fiberarrays at the non-perpendicular angle to the surface of the third wafer.19. A product of the process according to claim
 1. 20. A product of theprocess according to claim
 4. 21. A product of the process according toclaim
 7. 22. A product of the process according to claim
 12. 23. Themethod according to claim 1, wherein the energy sensitive material ofthe template backing layer is selected from the group consisting ofphotoresist material, anti-reflective material, and adhesive material.24. The method according to claim 4, wherein the energy sensitivematerial of the template structure is selected from the group consistingof photoresist material, anti-reflective material, and adhesivematerial.
 25. The method according to claim 12, wherein the energysensitive material of the first layer is selected from the groupconsisting of photoresist material, anti-reflective material, andadhesive material.
 26. The method according to claim 4, wherein the stepof providing a template structure formed from energy sensitive materialproduct having fiber arrays selected from the group consisting of (i)polymer micro-scale fibers, (ii) polymer nano-scale fibers, and (iii)polymer micro- and nanoscale fibers further comprises the steps of:forming a template backing layer of energy sensitive material on abacking material; exposing the template backing layer to a predeterminedpattern of energy from an energy source; forming a template layer ofenergy sensitive material on the template backing layer; applying a maskto the template layer; exposing the template layer to an energy source;removing the mask and a portion of the template layer not exposed to theenergy source to create a pattern of exposed energy sensitive materialfiber arrays forming the template structure, wherein the pattern ofexposed energy sensitive material fiber arrays is selected from thegroup consisting of (i) polymer micro-scale fibers, (ii) polymernano-scale fibers, and (iii) polymer micro- and nanoscale fibers.
 27. Amethod of forming a negative mold product having polymer micro- and/ornano-fiber arrays, comprising the steps performed in the followingsequence: forming a mold backing layer of energy sensitive material on abacking material; exposing the mold backing layer to a predeterminedpattern of energy from an energy source; forming a mold layer of energysensitive material on the mold backing layer; applying a mask to themold layer; exposing the mold layer to an energy source; and removingthe mask and a portion of the mold layer not exposed to the energysource to create a pattern of exposed energy sensitive material fiberarrays forming the negative mold product, wherein the pattern of exposedenergy sensitive material fiber arrays is selected from the groupconsisting of (i) polymer micro-scale fibers, (ii) polymer nano-scalefibers, and (iii) polymer micro- and nanoscale fibers.
 28. The methodaccording to claim 27, wherein the fiber arrays of the negative moldproduct are oriented at a perpendicular angle to a surface of a templatebacking layer of the template structure, wherein tip surfaces of thefiber arrays are flat and substantially parallel with the surface of thetemplate backing layer.
 29. The method according to claim 27, whereinthe fiber arrays of the template structure are oriented at anon-perpendicular angle to a surface of a template backing layer of thetemplate structure, wherein tip surfaces of the fiber arrays are flatand substantially parallel with the surface of the template backinglayer.
 30. The method according to claim 1, wherein the backing materialis a substrate.
 31. The method according to claim 1, wherein the backingmaterial is a coating.
 32. The method according to claim 12, wherein thebacking material is a substrate.
 33. The method according to claim 12,wherein the backing material is a coating.
 34. The method according toclaim 26, wherein the backing material is a substrate.
 35. The methodaccording to claim 26, wherein the backing material is a coating. 36.The method according to claim 27, wherein the backing material is asubstrate.
 37. The method according to claim 27, wherein the backingmaterial is a coating.
 38. A method of manufacturing a product havingpolymer micro- and/or nano-fibers comprising the steps of: forming a dryadhesive backing layer of energy sensitive material on a backingmaterial; exposing the dry adhesive backing layer to a predeterminedpattern of energy from an energy source; forming a dry adhesive layer ofenergy sensitive material on the dry adhesive backing layer; applying amask to the dry adhesive layer; exposing the dry adhesive layer to anenergy source; and removing the mask and a portion of the dry adhesivelayer not exposed to the energy source to create a pattern of exposedenergy sensitive material fiber arrays forming the product, wherein thepattern of exposed energy sensitive material fiber arrays is selectedfrom the group consisting of (i) polymer micro-scale fibers, (ii)polymer nano-scale fibers, and (iii) polymer micro- and nanoscalefibers.
 39. The method according to claim 1, wherein the templatestructure is a positive representation of a final product.
 40. Themethod according to claim 4, wherein the negative mold product is anegative representation of a final product.
 41. The method according toclaim 27, wherein the negative mold product is a negative representationof a final product.